The present invention relates to automatic electronic or "computer" flash systems. In particular, the present invention relates to an improved triggering circuit for use with automatic electronic flash systems having a remote sensor.
Automatic electronic flash systems include a light producing means, generally a flash tube, which is actuated to illuminate a scene being photographed. A light sensing means detects the scene illumination and produces a light termination or "quench" signal when sufficient light has been produced to properly expose a light sensitive film of an associated camera.
One advantageous flash system utilizes a remote light sensing means or "remote sensor" which is connected to the electronic flash unit by a two-wire cord or cable. Systems of this type are described in U.S. Pat. Nos. 3,714,443 by F. T. Ogawa; 3,737,721 by F. T. Ogawa; 3,793,556 by D. J. Wilwerding; 3,758,822 by D. J. Wilwerding; and 3,914,647 by B. Broekstra and D. J. Wilwerding. While these systems have been generally satisfactory, there has still existed a need for improved flash systems using a remote sensor.
In particular, an improved triggering circuit for electronic flash systems having a remote sensor is needed. Previous systems typically include a transistor which is in a conductive state prior to initiation of the flash. This transistor is connected between the gate and cathode of a silicon controlled rectifier (SCR). Closure of the contacts by the user causes the transistor to turn off which, in turn, turns on the SCR. When the SCR turns on, the triggering pulse is applied to the flash tube.
This prior art triggering circuit has an important disadvantage. The turning off of the transistor results in a gradual rise in the gate voltage rather than an abrupt rise. This gradual rise, which is termed "soft" gating, can result in destruction of the SCR since very large anode to cathode current can flow through just a portion of the SCR when it initially begins to turn on.